1. Field of the Invention
This invention relates to a magnetic fluid seal device to be used as a non-contact shaft sealing part in vacuum equipment such as ion plating apparatus and dry etching apparatus, and rotating devices handling gas and other rotating shaft parts. A magnetic fluid seal device of this sort is free from friction noise, and attains a high degree of air tightness in a simple structure, and withstands high speed rotations of over thousands of rpm, and hence it is used widely in rotary shafts parts, replacing mechanical seals.
2. Description of the Prior Art
In the conventional magnetic fluid seal device of this sort, plural pole pieces in a cylindrical form with a thick wall were disposed in a housing around a magnetic shaft across a magnet to form narrow gaps between the pole pieces and the magnetic shaft, and magnetic fluid was magnetically captured in these gaps. In this basic structure, generally in order to obtain wide and narrow gaps (labyrinth) between the pole pieces and the magnetic shaft, for example, annular projections were formed around the magnetic shaft, or block rings having fine undulations on the inner surface were used as pole pieces as disclosed in the U.S. Pat. No. 3,488,531.
In such a magnetic fluid seal, however, it was very uneconomical to form undulations around the shaft or fine asperities on the inside of the block rings with high precision, and it was structurally impossible to form multiple magnetic fluid capturing parts having a high magnetic flux density in the narrow gaps in the axial direction.
As a magnetic fluid seal device for solving such problems, the present applicant previously proposed the device of using disk packs obtained by alternately joining and adhering magnetic disks differing in the inside diameter of the pole pieces as disclosed in the Japanese Utility Model Publication No. 62-71473.
However, in the magnetic fluid seal device disclosed in this Publication No. 62-71473, it was sometimes impossible to capture the magnetic fluid securely between the magnetic shaft and the magnetic disk of the smaller inside diameter side, and a sufficient sealing effect could not be obtained.
It has been found that the above problem was caused because of two reasons, that is:
(a) If the gap between the magnetic disk with the smaller inside diameter and the outer circumference of the magnetic rotating element is larger than the thickness of the magnetic disk of the smaller inside diameter, the magnetic flux density between the magnetic disk and the magnetic rotating element, serving as the sealing part, is lowered. PA1 (b) When the magnetization of the magnetic fluid in the tiny gaps is not sufficient, the magnetic fluid cannot be captured securely between the magnetic disk of the smaller inside diameter and the magnetic shaft.
That is, due to causes (a) and (b), the differential pressure resistance characteristics between the primary side and secondary side of the magnetic fluid device are lowered, thereby making it impossible to maintain the desired high air tightness.